Which of the following reactions is an example of nuclear fusion?

Nuclear fusion is when light nuclei join to make a heavier one and release energy. Hydrogen nuclei combining to form helium is fusion. It is how the Sun makes its energy.

Uranium-235 splitting into barium and krypton is fission, the splitting of a heavy nucleus. Thorium decaying to radium is alpha decay. Carbon-14 beta decay is a nucleus changing a neutron to a proton. None of these join nuclei together.